1. Field of the Invention
Fuel injectors of internal combustion engines execute a stroke-controlled or pressure-controlled injection of highly pressurized fuel into the combustion chamber of an engine. In order to comply with current and future exhaust regulations for internal combustion engines, it has become necessary to execute multiple injections (preinjections, main injections, and secondary injections). The time interval between these individual injections should be as short as possible and should at the same time exert as little influence as possible on the subsequent injection. A pilot injection, which precedes the main injection phase and is intended for conditioning the combustion chamber should not influence a subsequent main injection phase with regard to the pressure increase relevant to emissions.
2. Prior Art
The subject of DE 196 50 865 A1 is a solenoid valve for controlling the fuel pressure in the control pressure chamber of an injection valve element, for example in common rail injection systems. The fuel pressure in the control pressure chamber is used to control the movement of a valve piston that opens or closes the injection openings of the injection valve. The solenoid has an electromagnet disposed in a housing part, a moving armature, and a control valve element that is moved by the armature, is acted on in the closing direction by a closing spring, and cooperates with a valve seat of the solenoid valve, thus controlling the flow of fuel out of the control pressure chamber. DE 197 08 104 A1 has also disclosed a solenoid valve of this kind for controlling the fuel pressure in the control pressure chamber of an injection valve.
In order to avoid the disadvantageous armature chatter that occurs in solenoid valves when they are triggered, the armatures of the solenoid valves according to DE 196 50 865 A1 and DE 197 08 104 A1 are embodied as two-part armatures. The armatures have an armature rod and an armature plate that is mounted in sliding fashion onto the armature rod. The use of two-part armatures reduces their effectively braked mass and therefore reduces the kinetic energy of the armature striking the valve seat and thus causing the armature chatter. A triggering of the solenoid valve only results in a definite injection quantity once the postoscillation of the armature plate has finished. It is therefore necessary to take steps to reduce the postoscillation of the armature plate. This is particularly necessary when short time intervals are required between a preinjection and main injection phase. In order to solve this problem, damping devices are used, which have a stationary part and a part that moves with the armature plate. The stationary part can be comprised of a maximum stroke stop, which limits the maximum travel length by which the armature plate can slide on the armature rod. The moving part is comprised of a protrusion that is provided on an armature plate and is oriented toward the stationary part. The maximum stroke stop can be constituted by the end surface of a sliding piece that guides the armature rod and is clamped in a stationary fashion in the housing of the solenoid valve or by a part such as a washer disposed in front of the sliding piece. When the armature plate approaches the maximum stroke stop, a hydraulic damping chamber is formed between the opposing end surfaces of the armature plate and the maximum stroke stop. The fuel contained in the damping chamber exerts a force that counteracts the movement of the armature plate, thus exerting a powerful damping action on the postoscillation of the armature plate.
The disadvantage of the solenoid valves according to DE 196 50 865 A1 and DE 197 08 104 A1 is the precise adjustment of the maximum sliding travel available to the armature plate on the armature rod. The maximum sliding travel, also referred to as maximum stroke, is adjusted by changing the maximum stroke washer, by adding spacers, or by machining down the maximum stroke stop. Since they require an iterative adjustment that must be carried out in steps, these embodiments are costly, are difficult to automate, and therefore extend the cycle times that the manufacture of such solenoid valves requires.
Stroke-controlled fuel injectors in current use for high-pressure injection systems with a high-pressure reservoir each have a throttle and an actuator that can be embodied as a magnet coil or as a piezoelectric actuator. These components, however, only permit the achievement of very low opening and closing speeds of an injection valve element, which can be embodied as a nozzle needle. In multiple injections, it is therefore not possible to use different needle opening speeds to influence the pressure increase, which is decisive with regard to emissions, in such a way that a pilot injection (PI) occurs very close to the main injection phase without influencing the subsequent injections in a functionally critical manner.